In the 63th conference of the 3rd Generation Partnership Project (3GPP) RAN1 for a cellular communication system, four coordinated multi-point transmission (CoMP,) schemes are defined. In a third CoMP scheme, at a transfer node that includes a Macro Site and a radio remote head (RRH) and is in a macro base station area, one cell ID is allocated to each RRH, respectively, and such a structure is similar to a system where multiple base stations coexist. However, in a fourth CoMP scheme, a same cell ID is shared by all transfer nodes that include a macro base station and a radio remote head and is in a macro base station area, and the architecture is also referred to as a distributed antenna system (DAS).
In a 3GPP LTE-Release-11 system, to further enhance the throughput of DL (Downlink) and UL (Uplink) data transmission, solutions such as DL CoMP, UL CoMP, DL MIMO enhancement (Downlink Multiple Input Multiple Output enhancement), and HetNet (Heterogeneous Network) are adopted to greatly enhance data throughput of radio cell DL/UL, throughput at the edge of the radio cell, and user experience at the edge, for example, QoS (Quality of Service). However, the capacity of the existing DL and UL signaling mechanism in an LTE Release-11 system is insufficient to support the enhanced DL and UL data throughput. Therefore, in the current 3GPP LTE conference, an enhanced DL and UL signaling mechanism is described and adopted to expand the DL and UL signaling capacity, so as to support the enhanced DL and UL data throughput. Specifically, the enhanced DL and UL signaling mechanism includes an enhanced PDCCH (ePDCCH, enhanced physical downlink control channel), an enhanced PUCCH (Physical uplink control channel), an enhanced PRACH, an enhanced TA (Timing advance), an enhanced PHICH (Physical HARQ indicator channel), an enhanced PCFICH (Physical control frame indicator channel), an enhanced PBCH (Physical broadcast channel), and so on. At present, key points of discussions in the 3GPP LTE conference is the ePDCCH, the enhanced PUCCH, and the enhanced DL TA and UL TA. Nevertheless, considering that in an LTE Release-8/9/10 system, an RRH mechanism is not adopted and the number of user equipments (UE) is limited, and a PRACH resource is sufficient for a limited number of UEs to access the macro base station, while in an LTE Release-11 system, especially, multiple RRHs are introduced in a CoMP solution 4, so that a coverage of the radio cell expands greatly and the number of serving UEs increases largely, which may result in the existing PRACH resource of the LTE Release-8/9/10 system is insufficient for a large number of UEs to access the macro base station and/or one and/or more RRHs. Therefore, the PRACH resource needs to be further expanded.
Random access is a most basic function of a cellular system, which makes it possible to establish a connection between a UE and a network. The random access is initiated and the resource is adopted randomly, and of course the access also succeeds randomly. A scheme of random access is as follows:
Random access in a contention-based mode: an initial access in an RRC_IDLE status; an initial access after an error occurs on a radio link; in an RRC_CONNECTED status, when there is uplink data transmission, for example, after an uplink is non-synchronized, or no PUCCH resource is used for sending a scheduling request message, that is, in this case, except for a random access manner, there is no other way to inform an eNB that the UE has uplink data that needs to be sent.
Random access in a non-contention-based mode: in the RRC_CONNECTED status, when there is downlink data transmission, the uplink is non-synchronized. The data transmission further requires confirmation in addition to reception, and therefore, if the uplink is non-synchronized, the eNB cannot guarantee that confirmation information of the UE can be received. In this case, the downlink is still synchronized, and therefore, the UE can be informed through a downlink message of a resource, for example, a preamble sequence (or referred to as a “PRACH sequence” or a “preamble”) and sending timing, that needs to be used for initiating random access. Because these resources are known to both parties, the system does not need to be accessed through a contention manner. For the random access in a switching process, during the switching process, a target eNB may inform the UE through a serving eNB of the resource that it can use; whether it is contention-based lies in whether in that case a terminal can intercept a downlink control channel information transferred by the eNB, so as to acquire a specific resource to be used for transmitting an uplink preamble, and definitely, the judgment is made by the eNB rather than deciding by the UE itself.
A process of initializing a random access process is as follows: The random access process may be triggered by a PDCCH order or a MAC (Media Access Control) sublayer. If one PDCCH transmission received by the UE includes one PDCCH order, the UE initiates one random access process. The PDCCH order or an RRC message indicates ra-PreambleIndex and ra-PRACH-MaskIndex information to inform the UE of the preamble sequence and a sending opportunity that the UE can use.
Before the random access process is initiated, the following information is provided:
The PRACH resource which is used for sending a random access preamble is prepared, which is indicated by prach-ConfigIndex; there is an available random access preamble, and two groups of random access preambles may be set at a MAC layer: Group B and Group A, which are used for indicating the size of an MSG3 (message 3) that is sent, respectively, the number of preamble sequences of Group B can be deduced and obtained from the following parameters, the number of preamble sequences of Group B=numberOfRA-Preambles−sizeOfRA-PreamblesGroupA. In SIB2, the foregoing two parameters are provided in the defined PRACH radio resource. If the preamble sequences of Group A are equal to the total random access preamble sequences, the UE knows that there is no preamble sequence of Group B. Serial numbers of the preamble sequences of Group A and Group B are as follows: [0 sizeOfRA-PreamblesGroupA−1] and [sizeOfRA-PreamblesGroupA numberOfRA-Preambles−1]. The UE select Group A or selects Group B depending on whether it is needed and a certain condition is satisfied. For example, the UE intends to carry a VoIP (Voice over Internet Protocol) packet in the MSG3 that is sent, naturally the needed resource is a bit large, so that when preamble sequences which are sent by the UE and received by the eNB belong to Group B, it allocates more resource to the UE to send the MSG3. If there is preamble sequences of Group B, because an MSG3 message corresponding to Group B is relatively large, some additional requirements must be satisfied, and whether the preamble sequences of Group A or Group B are selected depends on some values and a current UE power condition, where these values are messagePowerOffsetGroupB, messageSizeGroupA, the configured UE transmit power PCMAX, and a power offset between the preamble sequences and the MSG3. A window size parameter ra-ResponseWindowSize of receiving a random access response is acquired, and the UE intercepts during this window period whether the eNB return a response to it. This response carries the resource which is used for sending the MSG3 and is allocated to the UE by the eNB. Therefore, the window size is the time that the UE waits. If no response is received, the UE determines that the preamble it sent is not received by the eNB, and subsequent processing is needed, for example, a power ramping step powerRampingStep. It is assumed that an access process initiated in the foregoing fails, but the maximum number of attempts is not reached, the UE increases the power to send preambles next time to enhance an opportunity of successful sending. The number of times that the sending can be attempted is preambleTransMax, it is usually considered that the UE accesses the network when this number of times is exceeded, and at least it can be considered that the access fails and the access failure is reported to an upper-layer protocol layer. The eNB expects the received preamble sequence target power preambleInitialReceivedTargetPower, and interference is caused if this value is too high and the preamble sequences may be unable to be received if the value is too low. A preamble sequence format corresponds to the power offset, and at present there are five types of preamble sequences, and each format corresponds to one reference selection transmit power. The maximum number of times of MSG3 HARQ retransmission is maxHARQ-Msg3Tx. A contention resolution timer is mac-ContentionResolutionTimer.
Only one random access process is allowed at a certain moment. If the UE is in one random access process while receiving a new random access request at the same time, whether to continue the current process or to cancel the current process depends on the implementation of the UE, and then one new process is initiated according to the new request.
A PRACH access mechanism of the LTE-Release-8/9/10 system adopts 64 base sequences (ID: 0 to 63), which includes three PRACH sequences, that is, the foregoing PRACH sequences of Group A, PRACH sequences of Group B, and non-contention based PRACH sequences, so as to solve the problem that PRACHs of all UEs access the eNB. However, because an LTE-Release 11 and later versions adopt multiple RRHs (Remote Radio Head) to enhance the DL and UL data capacity of the system, it is caused that the PRACH resource is insufficient. Especially in the four schemes of CoMP solutions, multiple RRHs and MeNBs (Macro eNB, macro eNodeB) adopt a same Cell ID (cell identity), thereby resulting in significant increase of the number of UEs in the radio cell, and the original 64 IDs become far insufficient for a large number of UEs to access multiple RRHs or MeNBs. In this way, the PRACH contention is caused to be so excessively fierce that many UEs fail to access the network, thereby affecting user experience of UEs.